1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus such as a digital copier and a laser printer that includes the optical scanning device.
2. Description of the Related Art
Examples of deflectors used in conventional optical scanning devices for scanning a target surface with light include a polygon mirror and a galvanometer mirror. To achieve higher-resolution images and higher-speed printing, the mirror needs to be rotated at higher speed. This raises issues such as wear-out of bearings, generation of heat due to windage loss, and generation of noise, which puts a limit on high-speed scanning.
Recently, to address such issues, studies have been conducted on deflectors produced by use of silicon micromachining. Japanese Patent No. 2722314, for example, discloses a system in which a vibration mirror and a twisting member that supports the vibration mirror are integrally formed on a Si substrate.
According to the technology disclosed in Japanese Patent No. 2722314, the size of the mirror surface can be reduced, which leads to reduction in the overall size of the device. In addition, because resonance is used to creates reciprocal vibration, high-speed operations can be realized while suppressing noise and power consumption.
Japanese Patent No. 3445691, for example, discloses a technology in which a vibration mirror is provided in place of a polygon mirror.
Japanese Patent Application Laid-open No. 2003-98459 discloses a technology for applying the technology of Japanese Patent No. 3445691 to an optical scanning device in a tandem image forming apparatus. According to the technology disclosed in Japanese Patent Application Laid-open No. 2003-98459, several vibration mirrors arranged and they are driven at a common scanning frequency. The resonance frequencies, however, may not always coincide because of the moment of inertia of the vibrating units and the spring constant of the twisting members, which tend to vary due to the fluctuations in dimensions caused during the producing process.
In response to this problem, an example in which displacement can be detected is proposed (see, for example, Japanese Patent No. 2657769).
Because each vibration mirror is individually driven, if the resonance frequencies do not coincide with one another, scan line pitches become uneven, resulting in displacement of the scanning lines that occurs gradually from the starting point to the end point in the sub-scanning direction. In addition, if the center of each vibration amplitude does not coincide with others, scale factors, or dot densities, vary among areas that are divided along the main scanning direction. This causes displacements of registers and unevenness in density among images of different colors, resulting in color shifts and discoloration. Thus, the image quality is degraded.
As the scanning frequency increases in accordance with scanning speed enhancement, the scan angle may be unable to catch up with the speed. Another limit is that the variation amount of the rotation angle per unit time drastically decreases in accordance with sinusoidal vibration as the scan angle nears its peak. For this reason, only about a half the entire scan angle can be effectively used when trying to achieve even dot intervals on the scanned surface.
Moreover, it is preferable to have a smaller image formation spot diameter to bring the shape of the latent image potential distribution closer to a rectangular and thus improve the resolution and maintain the evenness of the dot diameters. However, a gauss beam, in general, has image formation properties that ω0/ω is proportionate to the focus distance f of the image formation lens, where the diameter of beam incident on the image formation lens is represented as ω0 and the diameter of the image formation spot is represented as ω. This means that, when the angle of field becomes small with an insufficient scan angle, the focus distance f of the image formation lens inevitably becomes greater. To shrink the spot, the diameter of the beam ω0 needs to be increased, which means the mirror surface needs to be increased. For this reason, the situation is becoming more difficult to ensure a sufficient scan angle.
FIG. 18 is a perspective of a typical movable mirror.
A movable mirror shaped like a simple plate as shown in FIG. 18 will be considered. The dimensions of the movable mirror are determined as 2r in width in a direction parallel to the rotational axis, d in width in a direction orthogonal to the rotational axis, and t in thickness. The dimensions of each twisting member are determined as h in length, and a in width. When the density of Si is ρ, and the material constant is G,the moment of inertia I=(4ρrdt/3)·r2spring constant K=(G/2h)·{at(a2+t2)/12}Thus, the resonance frequency f0 is:f0=(½π)·v(K/I)=(½π)·v{Gat(a2+t2)/24LI}
The length of the twisting member L is almost proportional to the scan angle θ, and thus the scan angle θ can be expressed by:θ=κ/I·f02 where κ is a constant  (1)This means that the scan angle θ is inverse proportional to the moment of inertia I, and to increase the resonance frequency f0, the moment of inertia I must be reduced, or otherwise the scan angle θ would decrease. In other words, if 2r, width in a direction parallel to the rotational axis, is simply increased, the scan angle θ would decrease in inverse proportion to the cube of the magnification factor.
On the other hand, the relationship between the torque T and the scan angle θ can be expressed by:θ=κ′·T/K where κ′ is a constant  (2)This means that the scan angle θ can maintained at a constant value by adjusting the torque T.
When such a mirror is to be used in an optical scanning device of a tandem image forming apparatus, it is necessary to arrange more than one mirrors and drive them at a common frequency.
With the conventional technology, fluctuations in resonance frequencies that appear by fluctuations in dimensions during the producing process can be minimized to some extent by screening or with the method as described above. It is still difficult, however, to make the resonance frequencies exactly match. Furthermore, because the resonance frequencies also vary in accordance with the spring constant that varies due to the temperature changes, the scan angle decrease caused by fluctuations in a corresponding resonance frequency has to be compensated by an increase in the torque.
In the current situation in which of an image formation spot having a smaller diameter is demanded as a higher image quality is required, the mirror surface size cannot help but be increased. Therefore, the measures that use an increased torque for compensation are becoming less effective.
For the above reasons, a system that does not require multiple vibration mirrors, or a system that needs a single vibration mirror, is now in demand. Nevertheless, individual control of the starting positions between the opposing stations, which is enabled only with multiple vibration mirrors, will no longer be possible with a single mirror. The single-mirror system needs to be configured to suppress image quality degradation including color shifts and discoloration, without depending on such a control even when the scan angle varies.